Magnetic tunnel junction (MTJ) devices, typically comprising a fixed magnetic layer and a free magnetic layer separated by a tunneling barrier layer, utilize a phenomenon known as tunneling magnetoresistance (TMR). For a structure including two ferromagnetic layers separated by a thin insulating tunnel layer, it is more likely that electrons will tunnel through the tunnel material layer when magnetizations of the two magnetic layers are in a parallel orientation than if they are not (non-parallel or antiparallel orientation). As such, the pMTJ can be switched between two states of electrical resistance, one state having a low resistance and one state with a high resistance. The greater the differential in resistance, the higher the TMR ratio: (RAP−Rp/Rp*100% where Rp and RAP are resistances for parallel and antiparallel alignment of the magnetizations, respectively. The higher the TMR ratio, the more readily a bit can be reliably stored in association with the pMTJ resistive state. The TMR ratio of a given pMTJ is therefore an important performance metric of a spin transfer torque memory (STTM) that employs an pMTJ stack.
For an STTM device, current-induced magnetization switching may be used to set the bit states. Polarization states of a first (free) ferromagnetic layer can be switched relative to a fixed polarization of the second (fixed) ferromagnetic layer via the spin transfer torque phenomenon, enabling states of the pMTJ to be set by application of current. Angular momentum (spin) of the electrons may be polarized through one or more structures and techniques (e.g., direct current, spin-hall effect, etc.). These spin-polarized electrons can transfer their spin angular momentum to the magnetization of the free layer and cause it to precess. As such, the magnetization of the free magnetic layer can be switched by a pulse of current (e.g., in about 1-10 nanoseconds) exceeding a certain critical value, while magnetization of the fixed magnetic layer remains unchanged as long as the current pulse is below some higher threshold associated with the fixed layer architecture.
MTJs with magnetic electrodes having a perpendicular (out of plane of substrate) magnetic easy axis have a potential for realizing higher density memory than in-plane variants. Generally, perpendicular magnetic anisotropy (PMA) can be achieved in the free magnetic layer through interfacial perpendicular anisotropy established by an adjacent layer, such as magnesium oxide (MgO), when the free magnetic layer is sufficiently thin. Thin layers however are often associated with a relatively low coercive field Hc. Techniques and structures that can increase Hc for a given magnetic layer thickness are therefore advantageous, for example to improve pMTJ stability.